Side-Scanning Sonar Principles. Side-scanning sonars (SSS) are designed primarily to create visual images of the sea floor and any objects laying thereon. The image is created by scanning the area of interest with an acoustic beam. A short acoustic pulse, typically on the order of millisecond is launched from the projector array, and it travels away from the array at the sound propagation velocity of the medium, typically about 1500 m/s in sea water. The sea floor and any objects in the sound field reflect a portion of the sound back to the hydrophone array where it is sensed. The two-way travel time of the echoes provides a very accurate measure of the distance to the reflecting objects in a direction normal to the major axis of the acoustic array, called the cross-track direction. Scanning in the along-track direction is accomplished by physically moving the array while periodically emitting new acoustic pulses. The result is a two-dimensional map of the reflecting objects in the scanned area. The echo amplitudes recorded are related to the size, shape, orientation, and acoustic impedance of the reflecting objects in each spatial location.
The shape of the transmitted acoustic field (i.e., the projector beam pattern) as well as the shape of the receiver array pattern (and more particularly, the shape of their intersection) must be controlled very carefully to produce accurate images. There is a significant number of different approaches to their design, but all have quite broad beams in the vertical plane (typically around 90 degrees centered at 45 degrees below the horizon), and quite narrow beams in the horizontal plane (because the latter defines the sonar resolution in a along-track direction). Especially in very high resolution arrays, it is necessary to measure and correct the inter-channel differences in amplitude and phase due either to hydrophone element placement inaccuracies or electronic mismatches.
In fact, most modern side-scanning sonars produce extremely narrow search beams, on the order of 0.1 degrees, in the along-track direction, and this creates extreme difficulties in the array measurement and calibration process. The hydrophone array itself is nearly always operated in the near field, or Fresnel zone, where range &lt;&lt;L.sup.2 /.lambda., and where L is the total array length and .lambda. is the acoustic wavelength), which means it must be focused to produce useful beams. The depth of field is usually so short that the focal distance must be changed for different target distances. This is usually accomplished by subdividing the receiving array into a number of discrete transducer elements and electronically introducing a time delay or phase shift into the received signal that focuses the beam dynamically as the pulse travels. Additionally, side-scan sonars, unlike sector-scan sonars, must change their beam width as the pulse travels so as to maintain a constant along-track resolution at all ranges. This is usually accomplished by electronically changing the length of the hydrophone according to ##EQU1## where N.sub.e is the number of array element signals summed to form the beam, R is range to target, d is the element spacing, and .DELTA.x is the desired along-track resolution. This also requires that the array be divided into discrete elements. In general, it is necessary to space the elements at one half wavelength intervals (the spatial equivalent of the Nyquist sampling theorem) in order to prevent spatial aliasing (i.e., grating lobes). However, it is also desirable to minimize the number of elements used because each one requires an analog receiver channel, and a typical array may be several hundred wavelengths long. Fortunately, because the SSS projector only irradiates a finite sector, it is permissible to use spacings much greater than a wavelength. In fact, the spacing is usually chosen as great as possible without allowing receiver array grating lobes into the projector's sound beam under worst case conditions, which turns out, surprisingly, to be at the minimum desired operating range.
The result is that each element is many wavelengths long and thus has a fairly narrow beam pattern itself. This presents the problem shown in FIG. 1. The preferred method of measuring the relative phase and amplitude response of each element is to place an acoustic source on a line passing through the center of the array at a distance sufficient that all the elements have the source point within their 3 dB field of view and use a single acoustic pulse for the measurement. The required separation distance is frequently quite large, on the order of a hundred meters, so that very few test facilities are of sufficient size to accommodate such tests and even fewer are equipped with appropriate positioning equipment. As an example, a two meter long array comprised of 30 wavelength elements requires a target distance of at least 57 m from the array, and positioning accuracy of a few centimeters in the along-track direction.
The receive array beam pattern is the spatial equivalent of the system impulse response: it is necessary and sufficient to completely specify the system transfer function and thus to allow performance predictions for any operating environment of interest. It is, therefore, of great importance to obtain a valid and accurate measurement of the pattern, and yet it is almost never done on SSS arrays because of the difficulty of the measurement.
Sector-Scanning Sonars. Historically, the majority of sonars have been sector-scanning. Unlike side-scanners that scan laterally by translational movement, sector-scanners scan in bearing by rotational movement. Conventional beam pattern measurement equipments were designed to provide rotational beam patterns for sector-scanning sonars by rotating the array of interest while receiving pulses from a calibrated projector at fairly short distances and recording the amplitude versus bearing angle. This is entirely appropriate for recording short range patterns of sector scan sonars, i.e., those producing constant angular resolution rather than constant along-track resolution, having modest resolution on the order of one or more degrees. If longer range patterns are required, the speed of rotation must be reduced to accommodate the longer inter-ping time. Rotators of conventional design tend to stall at slow speeds so that the sampling density in many cases becomes too low to produce useful patterns. For high resolution side-scanning sonars this process is not usable. Because the SSS beam pattern is electronically varied considerably with range, it is necessary to obtain pattern measurements at several different ranges, particularly at minimum and maximum design ranges. As an example, to measure the one-way receive pattern of a 0.1 degree sonar array at a range of 100 m, and to obtain a sampling density of at least 10 sample points in the main lobe to provide a smooth pattern requires a maximum rotation rate of 0.15 deg/s and accurately spaced sampling intervals of 0.02 degrees; well beyond the capability of conventional rotators and angle measuring equipment.
Other Prior Art. Only one alternate approach is known to have been used: a long (15 m) horizontal rail facility was built at the U.S. Navy Coastal Systems Station (CSS) in 1978 and attached to the old Hathaway Bridge in Saint Andrews Bay, Florida. This unit had the capability to translate, rather than rotate, a hydrophone array horizontally across a distance of about 12 meters. The translator was built to very exacting flatness and linearity specifications, and was an excellent appliance for the purpose of measuring SSS patterns. Unfortunately, it was very expensive and required a very elaborate mounting structure, and because the bridge has since been demolished, there is presently no way to deploy it. One disadvantage is that, because it used a horizontal translator, it also required a very bulky and complex mechanism to lower the entire rail and carriage system into the water for measurements and raise it to attach or remove the array under test.